Material for preventing crevice corrosion

ABSTRACT

SACRIFICING ANODE FOR PREVENTING CREVICE CORROSION TO BE CAUSED IN STANLESS STEEL CONTAINER OR BOILER IS CONSTITUTED BY COATING A CORE WIRE OF ROD-SHAPED CHROMIUM SERIES OR CHROMIUM-NICKEL SERIES STAINLESS STEEL WITH ALUMINUM OR ALUMINUM ALLOY, THE RATIO OF THE THICKNESS OF SAID COATED LAYER TO THE DIAMETER OF SAID CORE WIRE BEING 0.5 TO 10.

Aug. 20, 1974 YoswwsA IIV-U'ZUTANI: HAL f 3,830,

MATLRIAL FOR PREVENTING CREVICE CORROSION Filed May 24, 1971 I 2Sheets-Sp t 1 m 3 E I u 5 l5 Aug. 20, 1974 YOSHIHISA Ml ZUTANl ErAL3,830,720

MATLRIAL FOR PREVENTING CREVICE CORROSION Filed Ray 24, 1971 O 2Sheets-Sheet 2 Fig- United States Patent 3,830,720 MATERIAL FORPREVENTING CREVICE CORROSION Yoshihisa Mizutani, and Isao Ohba,Nishinomiya, and Takeshi Misaki, Ikeda, and Harumi Satoh, Hirakata,Japan, assignors to Nippon Yakin Kogyo Company Limited, Tokyo, JapanFiled May 24, 1971, Ser. No. 146,419 Claims priority, application Japan,May 30, 1970, 45/46,003 Int. Cl. C23f 13/00 US. Cl. 204-197 4 ClaimsABSTRACT OF THE DISCLOSURE Sacrificing anode for preventing crevicecorrosion to be caused in stainless steel container or boiler isconsti-' tuted by coating a core wire of rod-shaped chromium series orchromium-nickel series stainless steel with aluminum or aluminum alloy,the ratio of the thickness of (said coated layer to the diameter of saidcore wire being The present invention relates to a sacrificing anode forpreventing crevice corrosion, and particularly, a sacrificing anode tobe applied for preventing crevice corrosion in a stainless steelcontainer or tank. Recently, water boilers and heat exchangers used inchemical plant and home have used stainless steel in View of corrosionprevention, but crevice corrosion of stainless steel tank often occursin water environment containing an impurity (Cl ion, etc.). Namely, thestainless steel container or tank is manufactured by bending and weldinga stainless steel plate, and the chlorine ion in water concentrates inthe gaps and the concentration of oxygen in the gaps decreases.Consequently, an electrochemical crevice corro- SlOll occurs in the gapsas Well as pitting and stress corrosion cracking. This crevice corrosionis an important problem in the above described apparatus.

The object of the present invention is to solve the above describedfault and we pronounce a sacrificing anode capable of preventing thecrevice corrosion in the crevice in stainless steel container or tank.That is, the above described crevice corrosion is an important problemin Water boilers or heat exchangers made of stainless steel plate and asthe means for solving this problem, a variety of processes forpreventing the corrosion have been proposed.

The sacrificing anode according to the present invention is a compositemetal rod in which stainless steel is used as a core wire and the covermetal is aluminum or aluminum alloy. This composite metal uses stainlesssteel wire as the core wire and therefore jig to be used for fitting acommercial sacrificing anode is not needed and such wire can be easilyfitted by an easy method, for example, spot welding. Furthermore, ifaluminum alloy wire (uncomposite metal) is used, alumina is formed onthe surface and the contact resistance increases and an efficiency ofcorrosion preventing current is low.

In this composite metal according to the present invention, stainlesssteel is used in the core wire, so that the contact resistance decreasesand the efiiciency of corrosion preventing current is high.

When an uncomposite metal is used, corrosion occurs locally andcommercial anode is separated from the fitted position and the corrosionpreventing effect lowers, while when the sacrificing anode of thepresent invention is used, such fault does not occur and an excellentresult can be obtained.

The shape of this composite metal may be triangle, square, star and thelike other than circular shape depend- "ice ing upon the shape of theportion to prevent corrosion attack.

The manufacture by drawing is most preferable and for the purpose, thesuitable dimension of the crosssection to extrude into cylindrical orrod shaped sacrificing anode has been studied and it has been found thatwhen the ratio of the thickness of aluminum (coating metal) to thediameter of the cross-section of chromium series or chromium-nickelseries stainless steel core wire is 0.5 to 10, the drawing can beeffected most preferably. When said ratio is more than 10, the dimensionof the cross-section of the composite metal wire is too large and thedrawing cannot be effected satisfactorily, while when said ratio is lessthan 0.5, the thickness of aluminum material layer is too thin and thecorrosion preventing effect is lost in a short time.

The present invention will be explained in more detail hereinafter.

For a better understanding of the invention, reference is taken to theaccompanying drawings, wherein:

FIG. '1 is a perspective view of the sacrificing anode of the presentinvention;

FIG. 2 is a cross-sectional view of a water boiler fitting thesacrificing anode of the present invention;

FIG. 3 is a cross-sectional view of the boiler shown in FIG. 2 taken online A-A; and

FIG. 4 is an enlarged view of the gap portion in 'FIG. 2.

Referring to FIG. 1, when the tank is made of l8-8 series stainlesssteel, 18-8 series stainless steel wire 1 is used as the core wire andis coated with aluminum or aluminum alloy 2. (In this case, as mentionedabove, the rod-shaped material to be used as the core wire must be thesame quality as the material composing the container or boiler and whenthe boiler is made of 1'8-8 series stainless steel plate, the core wireis composed of 18-8 series stainless steel. Because, if the container orboiler and the core metal are not the same metal, the core metal is notfavorably welded to the container or boiler and further if there is thestainless steel core wire 1, even if aluminum coating material 2 iscorroded locally, the eifect can be kept by the presence of the corewire '1 until the aluminum coating material is completely consumed.

In order to combine the aluminum material 2 and 18-8 series stainlesssteel core wire 1, it is most preferable that molten aluminum materialis casted around the 18-8 series stainless steel core wire to form acast. Then the resulting cast is drawn to form the composite sacrificinganode 3 as shown in FIG. 1. The ratio (T/D) of the thickness (T) of thealuminum coating layer 2 to the diameter (D) of 18-8 series stainlesssteel core wire is about 0.5 to 10 based on the above described reasonand within this range, the ratio is selected properly considering thecondition of the loation to be fitted and the like.

FIGS. 2 and 3 show the water boiler wherein the sacrificing anode of thepresent invention is fitted on the boiler, the structure of which iswell known.

The fuel is ignited by a burner 4 and burned in a combustion chamber 5and the formed gas is discharged upwardly from the water boiler throughsmoke tubes 6 and 7, which are arranged concentrically in the casing 8.Hot water containing a sufficient amount of a boiler compound isintroduced into the casing 8 through an inlet 9 and circulated in thecasing 8 and during the circulation, the hot water is heated through thewall surface of the tubes 6 and 7 by the combustion gas. The heatedwater heats the fluid in a chamber 10 and flows out from an outlet 11and returns to the inlet 9. The chamber 10 containing the fluid to beheated is made of 18-8 series stainless steel and is constituted with acylinder 12, upper and lower covers 13, 14 and into the chamber 10, thefluid is charged from an inlet 16 and discharged from an outlet 15 whilebeing heated.

The sacrificing anode of the present invention is fitted in a gap of thewelded portion in the chamber 10 of the water boiler and the fittingmanner is shown in FIG. 4 by an enlarged view. That is, both the ends ofthe cylinder 12 are welded to the upper cover 13 and the lower cover 14and in the vicinity of the gap in the welded portion 18, both the endsof the core wire of the sacrificing anode are welded to the stainlesssteel cylinder 12 by TIG welding.

In order to confirm the effect of the sacrificing anode according to thepresent invention, a practical test for comparing water boilers fittedwith the sacrificing anode according to the present invention with otherwater boilers was effected. Water boilers used as a control have thesame structure as the water boiler shown in FIGS. 2 and 3. In the test,water is circulated and heated in the water boiler and the crevicecorrosion was examined. The obtained result is shown in the following.

EXPERIMENT NO. 1

Water boiler: Water boiler is fitted with no sacrificing anode.

Test period: 1 month Corrosion: Pitting and stress corrosion crackingwere observed at the gap 17 in the chamber 10 shown in FIG. 2.Particularly, the cylinder 12 was perforated by corrosion. Suchcorrosion was also formed in 3 portions on the upper cover 13.

, EXPERIMENT NO. 2

Water boiler: Water boiler is fitted with the sacrificing anodeaccording to the present invention. The coating material 2 is that shownin JIS 1100 (Aluminum, purity 99.00%, upper; ASTM 990A; B.S. 1B; DIN A199), and the core wire 1 is 18-8 (2) stainless steel.

Test period: 3 years Corrosion: There were no problems after the boilerwas used for 3 years. Particularly, after the boiler was used for 3years, the potential difference was 300 mv. at a Water temperature of 85to 90 C. This value was substantially the same as the value just beforethe boiler was used.

EXPERIMENT NO. 3

Water boiler: Water boiler is fitted with the sacrificing anodeaccording to the present invention. The coating material 2 is that shownin HS 5056 (Aluminum alloy; mg 5.2%, Mn 0.1%, Cr 0.1%, balance Al; B.S.N6; DIN AlMg and the core wire 1 is 18-8 (2) stainless steel.

Test period: 3 years Corrosion: There were no problems after the boilerwas used for 3 years. Particularly, after the boiler was used for 3years, the potential difference was 300 mv. at a water temperature of 85to 90 C. This value was substantially the same as the value just beforethe boiler was used. i

EXPERIMENT NO. 4

Water boiler: Water boiler is fitted with an aluminum sacrificing anodein the center of the chamber 10.

Test period: 6 months Corrosion: There were no problems after the boilerwas used for 1 month. However, after 6 months, corrosion similar to thatin Experiment No. 1 was formed at the gap 17 in FIG. 2, and the cylinder12 is perforated by COITOSIOD.

EXPERIMENT NO. 5

Water boiler: Sacrificing anode consisting of mild steel core wirecoated with aluminum coating material is arranged near the gap, and themild steel core wire is welded to the stainless steel boiler.

Test period: 13 months Corrosion: The result is superior to ExperimentNos. 1 and 4. However, after 13 months, aluminum on the surface of thesacrificing anode was consumed and the corrosion occurred at the gap 17in FIG. 2. Welded portion of the mild steel core wire to the stainlesssteel chamber 10 was corroded.

Test conditions of the above described 'Experment No. 1 to 5 are asfollows.

Operation time of water boiler: 8 hours/ day Temperature of circulatingwater containing a boiler compound: 90 to 95 C. at the outlet 11 of FIG.2.

In the above described test, the water boiler was operated for 8 hoursper day, and the corroded state of the boiler was observed. Thetemperature of circulating water at the inlet 9 was to C. and that atthe outlet 11 was 90 to C. The temperature of water to be heated at theinlet was room temperature and that at the outlet was 80 to 85 C. Thewater to be heated was adjusted to a Cl ion concentration of p.p.m. byadding NaCl to city water.

As seen from the above description and the above described test, crevicecorrosion often occurs at the gap of the welded portion in the stainlesssteel container or boiler, and the corrosion develops rapidly dependingupon the water to be treated and the life of the container or boiler isshortened. In such a case, it is effective for preventing the crevicecorrosion to fit aluminum or aluminum alloy sacrificing anode. However,such means still has defects as shown in the above Experiment No. 4.Moveover, particular caution is required in order to fit the aluminum oraluminum alloy single component sacrificing anode to the gap, because.aluminum and aluminum alloy themselves are dissolved out partly orwholly and broken, whereby they are separated from the container orboiler.

Further, when a sacrificing anode is prepared by surrounding a mildsteel core wire with aluminum or aluminum alloy, and the core wire iswelded to the tank of water boiler (Experiment No. 5), the sacrificinganode cannot be easily and strongly fitted to the tank by the welding,and further the mild steel core wire itself and the welded portion arepreferentially corroded.

On the other hand, when stainless steel is used as a core wire insteadof mild steel, the above described drawbacks which occur in the use ofmild steel, can be obviated. That is, in the sacrificing anode accordingto the present invention, stainless steel is used as the core wire, andaluminum or aluminum alloy is used as the coating material. Thesacrificing anode of the present invention is produced by adhering thecoatitng material around the core wire by mechanical means, such aspressing and the like; by casting the coating material around the corewire and then drawing the resulting cast; or by coating the surface ofthe core wire with the coating material and then drawing the resultingmass. The thus obtained sacrificing anode can be fitted to the stainlesssteel boiler by welding the core wire to the boiler, and consequentlythe welded portion is not corroded and has a high efi'iciency ofcorrosion preventing current.

However, when the aluminum or aluminum alloy coating material is adheredaround the sainless steel core wire by mechanical means, such aspressing, the potential difference between the sacrificing anode and theboiler is apt to lower. Because, fine gaps are formed between the corewire and the coating material and a contact resistance appears betweenthe core wire and the coating material due to the presence of Cl ions inthe water to be heated. Accordingly, preferable sacrificing anode is oneproduced by casting the aluminum or aluminum alloy coating materialaround the stainless steel core Wire and then drawing the resulting castor by coating the surface of the stainless steel core wire with thealuminum or aluminum alloy coating material and then drawing theresulting mass.

As described above, since the core wire of the sacrificing anodeaccording to the present invention is made of the same series stainlesssteel as that of the boiler, even if the sacrificing anode is fitted tothe boiler by spot welding, corrosion due to the potential difierencecaused in the boiler does not occur, and the boiler can be used for along period of time. For example, when a mild steel core wire is used asshown in the above described Experiment No. 5, corrosion occurs in thewelded portion of the mild steel core wire to the stainless steelboiler. Particularly, when mild steel and stainless steel are welded,the mechanical property and the corrosion resistance of the welded metalare naturally deteriorated.

"What is claimed is:

1. An apparatus having crevice corrosion preventing means comprising: ahousing adapted to being used in a water environment and composed of achromium series or chromium-nickel series stainless steel, said housinghaving at least one crevice or gap therein subjected to corrosion; and asacrificing anode consisting of a core wire made from the same stainlesssteel as said housing and a surrounding sheath of aluminum or aluminumalloy, said sacrificing anode being located in the vicinity of saidcrevice or gap in the housing, said stainless steel core wire being incontact with the stainless steel surface of the 6 housing, the ratio ofthe thickness of said sheath to the diameter of said core wire beingabout 0.5 to 10.

2. The apparatus of claim 1 in which said housing is a water boiler.

3. The apparatus of claim 1 in which said housing is a heat exchanger.

4. The apparatus of claim- 1 in which the molten aluminum or aluminumalloy is cast around the stainless steel core wire and then theresulting cast is drawn to obtain the stainless steel core wire coatedwith aluminum or aluminum alloy.

References Cited UNITED STATES PATENTS 2,478,478 8/1949 Grebe 204-1972,619,455 11/1952 Harris et a1 204-197 2,752,265 6/1956 Whitfield29-1962 X 867,659 10/ 1907 Hoopes et al 29-1962 X 3,425,925 2/1969Fleishman 204-197 X 2,762,771 9/1956 Preiser 29-1962 X 3,400,450 9/ 1968Nock, Jr. et a1. 29-1962 X ALLEN B. CURTIS, Primary Examiner US. Cl.X.R. 29-1962

